For many applications, including optical communication systems, photodetectors are required. Silicon is a widely used material for photodetectors but it has a bandgap of approximately 1.12 eV which restricts its utility to those applications in which radiation having a wavelength less than approximately 1.0 .mu.m will be detected. Accordingly, for detection of radiation at wavelengths longer than 1.0 .mu.m, other materials must be used. Materials commonly used include Ge and Group III-V compound semiconductors such as InGaAs.
For both p-i-n photodiodes and avalanche photodetectors, germanium is a less than ideal semiconductor because one should use its direct bandgap, 0.8 eV, for the absorption of radiation while the relatively small indirect bandgap, 0.66 eV, leads to large dark currents in typical device configurations. Additionally, because the ratio of the ionization coefficients is approximately 1.0, the rates at which the types of carriers ionize are not significantly different. This produces an intrinsically high noise level in an avalanche gain operating mode. As is well known to those skilled in the art, the lowest noise avalanche photodetectors arise when one type of carrier ionizes at a rate much greater than the other type of carrier, i.e., the ratio of the ionization coefficients differs significantly from 1.0. Group III-V compound semiconductors are not ideal for avalanche photodetectors because they also have a relatively small ratio of the ionization coefficients.
One approach to alleviating these problems in avalanche photodetectors involves the use of separate absorption and multiplication regions. The incident light is absorbed in a relatively small bandgap region and avalanche multiplication occurs in a relatively large bandgap region. One such photodetector is described in U.S. Pat. No. 4,212,019, issued on July 8, 1980 to Wataze et al. In one embodiment, his Example 3, the multiplication region comprised a p-type silicon layer and the absorption region comprised a p-type Ge.sub.x Si.sub.1-x layer. In another embodiment which is depicted in his FIG. 2, the multiplication and absorption regions are not clearly defined but rather, the composition of the Ge.sub.x Si.sub.1-x region is gradually varied. The detailed description states that the composition varies from pure Ge at the edge of the absorption region to pure Si at the edge of the multiplication region.
However, a detailed consideration of this disclosure by one skilled in the art reveals that the devices described are not suitable for use as photodetectors at wavelengths longer than approximately 1.2 .mu.m. In particular, they are not suitable for use as photodetectors in the 1.3 to 1.6 .mu.m wavelength range presently of interest for optical communication systems using silica-based fibers. This range is of interest because it includes the regions of lowest loss and minimum dispersion in the fiber. The limited utility, with respect to wavelength, of the avalanche photodetector arises because Ge and Si are indirect bandgap materials and a relatively thick GeSi absorbing layer is required for high quantum efficiency. In fact, an approximately 50 to 100 .mu.m layer will be required for most incident light to be absorbed. However, the structure disclosed cannot have a thick, high quality Ge.sub.x Si.sub.1-x absorbing layer on the silicon substrate because of the large lattice mismatch between the absorbing layer and the underlying silicon substrate. This lattice mismatch will inevitably result in a large number of defects, e.g., misfit dislocations, which will certainly preclude operation of the device as an avalanche photodetector. Additionally, even if the structure were fabricated without defects, it would not be useful for high speed communications applications because the photogenerated carriers would have to travel distances of the order of 50 .mu.m to reach the contacts. This would result in a response time of the order of a nanosecond.